Planetary Habitability

What Does it Mean?

In my last blog entry, I talked about oceans on ancient Mars and the evidence for them that planetary scientists are finding in craters on the Red Planet. All the questions about water on Mars really point to a big question about habitability — that is, a world’s capability of sustaining life.  There’s a sort of rote list of things that planetary scientists recite when it comes to assessing what a world has to support living beings. It has to have water, warmth, and organic material (food, essentially) for the life to exist. Those things are important for habitability.  So, if Mars had water in the distant past, and if it had warmth (from volcanism or heating from its core or if it had an atmosphere that could trap heat), then two of the three conditions for life would have been met. Food — organic material — would be a simple chemical problem to solve. Here on Earth, food for life ranges from the stuff you and I eat every day to the needs of such one-celled beings as bacteria that munch on sulfur.  Obviously, early Mars didn’t have gourmet delights that we could eat, but it could well have had plenty of delicacies for one-celled organisms.  So, the planet could have been habitable.  If we decided to live there in the future, it could still be termed habitable, but only just barely and we’d have to bring along habitats to take advantage of the barely habitable landscape. But, it could be done. And, it likely will be done.

So, we know Earth is habitable (still). And, Mars was and could be. But, what about Venus?  You know, that beautifully bright starlike object that’s making a nightly curtain call in our western skies after sunset?  Yeah, that one.  It’s gorgeous to look at, but if you landed on Venus, you’d fry in an instant, if you weren’t crushed to death first by the hellishly heavy and hot atmosphere.  So, how could this volcanic, sulfurous world be habitable?  Clearly it isn’t right now, but it may have been in the past.

What early Venus could have looked like. Could it have had a water ocean? Or was it more likely a molten world with a wet atmosphere? What happened to it? Credit: J. Whatmore.

Scientists at the European Space Agency are operators for the Venus Express orbiter, which has sent back data suggesting very strongly that early Venus could have water — perhaps even an ocean of it — and may have begun its planetary life as a much more Earthlike world. The spacecraft measured the escape of molecules of hydrogen and oxygen from Venus out to space.  The rate of escape of hydrogen is roughly twice that of oxygen, and this indicates that water is the source of these escaping materials. There’s also a tracer element called deuterium that also tells scientists that water has been escaping the planet. Deuterium is a heavy form of hydrogen, and it would have more difficulty escaping the planet’s gravitational pull. The presence of large amounts of it in the upper atmosphere of Venus tells us that water has also escaped and left the deuterium behind.

It’s probably unlikely that Venus had Earthlike oceans as shown in the artist’s concept above.  If it did have standing water, those pools and/or small oceans could have been formed when comets slammed into the molten surface.  If that happened, and if conditions were right, Venus could have been habitable for a short time in its early history. If that’s true, then it begs the question of whether life could have arisen on the planet, only to be snuffed out by Venus’s subsequent changing climate.  It’s an interesting idea and one that needs to be explored more.

However, the more likely scenario is that the newborn Venus had no oceans, but sported a very wet atmosphere overlying the molten surface.  Over time, sunlight broke the water molecules into hydrogen and oxygen (a process known as “photodissociation”). The newly freed gases fled to space, leaving behind the deuterium. The escape process cooled things down enough, and the surface cooled.

There’s still a lot of “ifs” in these scenarios, but the evidence for water (past and present) is strong, based on the Venus Express data.  It’s a good hint that the Venus we see today — hot, arid, miserable, and volcanic — wasn’t always this way.  And, it adds more to our store of knowledge about just when a planet can be habitable in its history — provided the conditions are right.

Stay tuned!

Wet Mars

The Proof is in the Craters

Lyot Crater on Mars, with lines indicating data swaths taken by the Mars Express OMEGA sensor, and NASA Mars Reconnaissance Orbiter CRISM instrument data.. The stars show where hydrated mineerals have been detected. Credits: NASA/ESA/JPL-Caltech/JHU-APL/IAS.

A neat piece of news caught my eye this week — an announcement from the European Space Agency that mineral studies of Mars taken by ESA’s Mars Express mission and NASA’s Mars Reconnaissance Mission show that liquid water was once very widespread on Mars.  The evidence lies inside craters spread around the planet, apparently just beneath the surface. It’s in the form of deposits of what are called hydrated silicates — minerals that have been in contact with water sometime in the past.

Lyot Crater (at left) was one of 91 impact craters the missions studied in a search for evidence of water. At least nine of the craters have strong evidence of hydrated silicates. Those minerals form in wet environments either on the surface or underground — and they have now been identified in both the north and south parts of Mars.

Why study craters?  Because the impacting objects (asteroid chunks, for example) punched down through the surface of the planet and exposed very ancient surface crust that would have been in contact with water. This means that water was widespread on the Martian surface sometime in the past. This is great news for scientists who are working to understand the role that water played on Mars early in its history. The presence of water means that conditions could have been favorable for life. It doesn’t prove that life existed on Mars — that takes other studies and will very likely require us to visit the planet to prove it for sure. But, the existence of water is a big thing.  There are hints of it all over Mars, not just in the hydrated silicates, but in the landforms that seem to be carved by the action of water.  This is a fascinating story that is still unfolding for planetary scientists. I, for one, think that we’ll find substantial reservoirs of water (probably locked away in subsurface aquifers and permafrost) on Mars when our first explorers set foot on that dry and dusty desert surface.